A photovoltaic device is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. Photovoltaic devices include photovoltaic cells that can generate and support an electric current when the cell is exposed to light, e.g., without electrical connection to an external voltage source. For example, when light such as sunlight is incident upon a material surface of the photovoltaic cell (e.g., a semiconductor material like silicon), the electrons present in the valence band of the material absorb the light energy, and thereby undergo an excitation by absorption of energy, causing a jump to the conduction band and becoming free electrons. The excited free electrons can diffuse, in which some can diffuse to a junction where they are accelerated into a different material, e.g., such as an electrical conductor, by a built-in potential. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current that can be used to power a load of the photovoltaic device.
Solar thermal energy (STE) systems and devices use solar energy to produce thermal energy (e.g., heat). STE systems and devices directly convert solar light energy to heat, rather than conversion to electricity as in photovoltaics. STE devices typically utilize solar energy much more efficiently than photovoltaics devices. Solar thermal energy systems can be classified as low-, medium-, or high-temperature systems based on the STE collectors used to convert solar energy to heat. Examples of low-temperature solar thermal energy systems include flat plate STE collectors used to heat swimming pools or provide space heating, in which the STE collectors use air or water as the medium to transfer the heat to their destination. Examples of medium-temperature solar thermal energy systems also include flat plate STE collectors, but can be used for heating water or air for larger residential and commercial use. High-temperature solar thermal energy systems include STE collectors that use mirrors or lenses to concentrate solar energy to produce heat energy to drive turbines or other mechanisms for electric power production.